Three-Dimensional Control-Gate Architecture For Single Poly EPROM Memory Devices Fabricated In Planar CMOS Technology

ABSTRACT

A capacitor for a single-poly floating gate device is fabricated on a semiconductor substrate along with low and high voltage transistors. Each transistor has a gate width greater than or equal to a minimum gate width of the associated process. A dielectric layer is formed over the substrate, and a patterned polysilicon structure is formed over the dielectric layer. The patterned polysilicon structure includes one or more narrow polysilicon lines, each having a width less than the minimum gate width. The LDD implants for low and high voltage transistors of the same conductivity type are allowed to enter the substrate, using the patterned polysilicon structure as a mask. A thermal drive-in cycle results in a continuous diffusion region that merges under the narrow polysilicon lines. Contacts formed adjacent to the narrow polysilicon lines and a metal-1 trace connected to the contacts may increase the resulting capacitance.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/326,582, entitled “Three Dimensional Control-Gate Architecture ForSingle Poly EPROM Memory Devices Fabricated In Planar CMOS Technology”filed Jan. 4, 2006.

FIELD OF THE INVENTION

The present invention relates to an EEPROM control gate fabricated usinga standard planar CMOS fabrication process.

RELATED ART

The main limitation of single-poly CMOS non-volatile semiconductormemory (NVSM) designs is a large cell area. One option to decrease thecell area is to form a control gate of the cell in the substrate. Asingle-poly non-volatile memory device using p-type and n-type dopedlayers underneath a single-poly gate layer has been described. Kwok etal., “An Innovative NVM Technology for Sub-0.25 um SOC Applications”,CASPA/CIE System-on-Chip (SOC) Symposium May 16, 1998, pp. 1-24.

Nae-In Lee et al. report an EEPROM device that uses a PMOS transistorstructure as the control gate for a NMOS EEPROM device. Lee et al.,“High-Performance EEPROM's Using N- and P-Channel Polysilicon Thin-FilmTransistors with Electron Cyclotron Resonance N20-Plasma Oxide”, IEEEElectron Device Letters, Vol. 20, No. 1, January 1999, pp. 15-17.Kumazaki (U.S. Pat. Nos. 6,818,943 and 6,489,650) reports an EEPROMsingle-poly device with a floating gate formed as a diffusion under apolysilicon layer, wherein the diffusion is formed by a special implantinto a silicon-on-insulator substrate prior to formation of the floatinggate.

The above-listed devices require special lithography and ion implantdoping steps that must be performed before the gate oxide is formed,thus adding complexity to the fabrication process.

The quality of the gate oxide that is thermally grown on heavily dopedsilicon regions is not adequate to provide a capacitive structure for anon-volatile memory cell.

The use of a PMOS or NMOS capacitor (transistor) as a control gate of anon-volatile memory cell results in a highly non-linear control gatecapacitance.

It would therefore be desirable to have a low cost NVSM embedded memorythat is easily integrated into a conventional planar CMOS process,preferably with no additional operations. It would further be desirableto have a high quality control gate capacitor structure with arelatively small size.

SUMMARY

Accordingly, the present invention provides a three-dimensionalcapacitor structure that combines various types of capacitance,including polysilicon-to-substrate capacitance, contact-to-polysiliconcapacitance and metal-to-polysilicon capacitance to obtain a largecapacitance in a small volume, thereby increasing the coupling ratio ofa control gate in a single-poly non-volatile semiconductor memorydesign.

In accordance with one embodiment, a three-dimensional capacitorstructure is fabricated on a semiconductor substrate using aconventional planar CMOS process, along with low and high voltagetransistors. Each of the low and high voltage transistors has a gatewith a width greater than or equal to a minimum gate width of the CMOSprocess. For example, low voltage transistors may have a gate width ofabout 0.18 microns, while high voltage transistors have a gate width ofabout 0.35 microns.

A capacitor structure having a p-type diffusion region is fabricated asfollows. A capacitor dielectric layer is formed over a first n-typeregion of the substrate. A patterned polysilicon structure is formedover the capacitor dielectric layer. The patterned polysilicon structureincludes one or more narrow polysilicon lines, each having a width lessthan the minimum gate width. For example, the narrow polysilicon linesof the patterned polysilicon structure may have a width of about 0.14microns. The patterned polysilicon structure also includes an extensionregion (having a width greater than or equal to the minimum gate width),which is used as the floating gate of the non-volatile memory device.

Gate dielectric layers and polysilicon gate electrodes are also formedfor the low voltage and high voltage transistors. A low voltage p-typelightly doped drain (LDD) implant is performed, thereby forming lightlydoped source/drain regions of the low voltage p-channel transistors. Thehigh voltage transistors are prevented from receiving the impurities ofthe low voltage p-type LDD implant. However, the patterned polysiliconstructure is exposed during the low voltage p-type LDD implant, suchthat the first region receives p-type impurities during this step.

A high voltage p-type lightly doped drain (LDD) implant is alsoperformed, thereby forming lightly doped source/drain regions of thehigh voltage p-channel transistors. The low voltage transistors areprevented from receiving the impurities of the high voltage p-type LDDimplant. However, the patterned polysilicon structure is exposed duringthe high voltage p-type LDD implant, such that the first region receivesp-type impurities during this step.

During a subsequent anneal, the p-type impurities laterally diffusewithin the first region of the substrate, and merge under the narrowpolysilicon lines of the patterned polysilicon structure, therebyforming a continuous diffusion region. This merging occurs due to therelatively narrow widths of the narrow polysilicon lines and therelatively high dopant concentration in the first region. A firstcapacitive element is formed by this continuous diffusion region, thecapacitor dielectric layer and the patterned polysilicon layer.

A pre-metal dielectric layer is subsequently formed over the patternedpolysilicon structure (and the low and high voltage transistors).Electrically conductive contacts, which extend through the pre-metaldielectric, are then formed. A first set of these contacts are locatedadjacent to the narrow polysilicon lines of the patterned polysiliconstructure. A second capacitive element is formed by the narrowpolysilicon lines, the pre-metal dielectric layer and the first set ofcontacts. Dielectric sidewall spacers may be formed adjacent to thenarrow polysilicon lines of the patterned polysilicon layer, therebyincreasing the capacitance of the second capacitive element.

A first metal layer (metal-1) trace may be formed over the pre-metaldielectric layer, in contact with the first set of contacts. In thiscase, a third capacitive element is formed by the metal-1 trace, thepre-metal dielectric layer and the narrow polysilicon lines. The threecapacitive elements are capable of providing a relatively highcapacitance in a relatively small volume.

Advantageously, the capacitor structure can be fabricated withoutrequiring additional masks or steps in a conventional planar CMOSprocess. In addition, because the continuous p-type diffusion region isformed after the capacitor dielectric layer and patterned polysiliconlayer have been formed, the capacitor dielectric layer will have arelatively high quality. That is, capacitor dielectric layer can bethermally grown silicon oxide, which is not adversely affected by thepresence of an underlying heavily doped p-type region.

In accordance with different variations of the present invention, somecapacitor structures may only include the first capacitive element orthe second capacitive element.

The present invention will be more fully understood in view of thefollowing description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a three dimensional capacitor structure, whichcan be used to implement a control gate of a single-poly non-volatilememory (NVM) cell, in accordance with one embodiment of the presentinvention.

FIG. 2 is a cross-sectional view of the three dimensional capacitorstructure of FIG. 1, along section line A-A.

FIG. 3 is an enlarged view of a portion of FIG. 2, which includes anopening in a patterned polysilicon structure.

FIG. 4 is a top view of a single-poly floating gate non-volatile memorycell, which implements the three-dimensional capacitor structure ofFIGS. 1-2 in accordance with one embodiment of the present invention.

FIG. 5 is a CMOS inverter transfer curve illustrating the response ofthe non-volatile memory cell of FIG. 4.

FIGS. 6A, 6B, 6C and 6D are cross-sectional views of the capacitorstructure of FIGS. 1-2, along with a low voltage PMOS transistor and ahigh voltage PMOS transistor, during various processing steps, inaccordance with one embodiment of the present invention.

FIGS. 7, 8, 9 and 10 are top views that illustrate variations of thecapacitor structure of FIGS. 1-2 in accordance with other embodiments ofthe present invention.

DETAILED DESCRIPTION

FIG. 1 is a top view of a capacitor structure 100, which can be used toimplement a three dimensional control gate of a single-poly non-volatilememory (NVM) cell, in accordance with one embodiment of the presentinvention. FIG. 2 is a cross-sectional view of capacitor structure 100along section line A-A of FIG. 1. As used herein, a single-poly NVM cellincludes any NVM cell fabricated with a single polysilicon gate layer.This single polysilicon gate layer is used to form the floating gates ofthe NVM cells, as well as the control gates of other transistorsfabricated on the same wafer. Capacitor structure 100 can be used, forexample, to implement the control gate of an electrically programmableerasable read only memory (EEPROM) cell or a compact flash memory cell.

Capacitor structure 100 includes p-type monocrystalline semiconductor(e.g., silicon) substrate 101, n-type well region 102, field dielectricregions 105, capacitor dielectric layer 108, dielectric sidewall spacers109, p-type semiconductor diffusion region 145, patterned polysiliconstructure 150, contacts 161-164, pre-metal dielectric layer 170 andfirst metal layer (metal-1) trace 180.

Field dielectric regions 105 (e.g., shallow trench isolation (STI)regions) are formed in n-well 102, thereby isolating capacitor structure100 from other active elements formed on substrate 101. Continuousp-type diffusion region 145 is located at the upper surface of n-well102, and is isolated by field dielectric regions 105. Capacitordielectric layer 108 is located over the upper surface of p-typediffusion region 145. Patterned polysilicon structure 150 is locatedover capacitor dielectric layer 108. Dielectric sidewall spacers 109 arelocated adjacent to the sidewalls of patterned polysilicon structure150. Pre-metal dielectric layer 170 is located over patternedpolysilicon structure 150, dielectric sidewall spacers 109 and capacitordielectric layer 108. Contacts 161-164 extend through pre-metaldielectric layer 170 and contact capacitor dielectric layer 108. Metal-1trace 180 is located over pre-metal dielectric layer 170, and contactsthe upper portions of contacts 161-164.

Patterned polysilicon structure 150 includes narrow polysilicon lines151-155 and wide polysilicon extension 156. In accordance with oneembodiment, wide polysilicon extension 156 forms the floating gate of asingle-poly NVM cell (not shown). Wide polysilicon extension 156 can bedesigned to have a width corresponding with the minimum gate width ofthe associated CMOS process. For example, wide polysilicon extension 156may have a width of 0.18 microns in a 0.18 micron CMOS process. Narrowpolysilicon lines 151-155 each have a width less than the minimum gatewidth of the associated CMOS process. For example, each of narrowpolysilicon lines 151-155 can have a width of about 0.14 microns in a0.18 micron CMOS process. As described in more detail below, the widthof narrow polysilicon lines 151-155 is selected to be small enough thatan impurity implanted through these lines will diffuse under these linesto form a continuous doped diffusion region (e.g., p-type diffusionregion 145). Narrow polysilicon lines 151-155 define openings 157 and158 in patterned polysilicon structure 150.

FIG. 3 is an enlarged view of a portion of FIG. 2, which includes theportion of patterned polysilicon structure 150 which defines opening158. This portion of patterned polysilicon structure 150, which includesnarrow polysilicon lines 151-152 and half of narrow polysilicon lines151-155, defines a square unit cell 300 of capacitor structure 100. Thecapacitance of this square unit cell 300 includes three main capacitiveelements C1, C2 and C3, which will now be described.

A first capacitive element C1 is formed by narrow polysilicon lines151-152, 154-155 and p-type diffusion region 145, which are separated bycapacitor dielectric layer 108. In a particular embodiment, capacitordielectric layer 108 is a high voltage gate oxide having a thickness ofabout 70 Angstroms. In this embodiment, the capacitance of capacitiveelement C1 is about 0.5 fF/μm2. The fringe capacitance of capacitiveelement C1 is about 0.05 fF/μm. Assuming that the square patternedpolysilicon structure has an area of about 0.45 μm2, capacitive elementC1 exhibits a capacitance of about 0.25 fF.

The second capacitive element C2 is formed by narrow polysilicon lines151-152, 154-155 and contact 162, which are separated by dielectricsidewall spacers 109 and pre-metal dielectric layer 170. In a particularembodiment, contact 162 is a tungsten plug having a diameter of about0.22 microns, patterned polysilicon structure 150 has a thickness ofabout 0.2 microns, and each of the dielectric sidewall spacers 109 issilicon nitride having a width of about 0.16 microns (i.e., the distancebetween patterned polysilicon structure 150 and contact 162 is about0.16 microns). In this embodiment, the capacitive element C2 exhibits acapacitance of about 0.2 fF.

The third capacitive element C3 is formed by metal-1 trace 180 andsquare patterned polysilicon structure, which are separated by pre-metaldielectric layer 170. In a particular embodiment, the capacitive elementC3 exhibits a capacitance of about 0.05 fF.

The total capacitance CT for one unit cell is equal to the sum of thecapacitances of capacitive elements C1, C2 and C3. In the describedexample, the capacitance CT is approximately equal to 0.5 fF (i.e., 0.25fF+0.20 fF+0.05 fF). Because capacitor structure 100 includes two unitcells, the total capacitance of capacitor structure 100 is approximatelyequal to 1.0 fF.

Advantageously, the total capacitance CT includes capacitive elements C1and C2, which exhibit the same order capacitance and have a weakdependence on voltage. Misalignment of a contact 161-164 with respect tothe walls of the patterned polysilicon structure 150 is not significant,because the system is self-compensating. That is, a smaller distancebetween a contact and one wall of the patterned polysilicon structure150 results in a larger distance between the contact and another wall ofthe patterned polysilicon structure 150. As a result, the influence ofprocess variations on total capacitance CT is minimized.

In the foregoing manner, capacitor structure 100 provides a capacitancethat can be used to implement a control gate non-volatile memory cell.

FIG. 4 is a top view of a single-poly floating gate non-volatile memorycell 400, which implements capacitor structure 100 in accordance withone embodiment of the present invention. NVM cell 400 includes capacitorstructure 100, p-channel field effect transistor 410, n-channel fieldeffect transistor 420, and select transistor 450. The layout of selecttransistor 450 is not shown for purposes of clarity. NVM cell 400 isgenerally described (with a different capacitor structure) in U.S. Pat.No. 6,788,576.

P-channel transistor 410 is formed in an n-well region 415, andn-channel NVM transistor 420 is formed in a p-well region 425. P-channeltransistor 410 includes a p-type drain region 411 and a p-type sourceregion 412. N-channel transistor 420 includes an n-type drain region 413and an n-type source region 414. Extension region 156 of patternedpolysilicon structure 150 forms the gate of both p-channel transistor410 and n-channel transistor 420. Metal bit lines BL1 and BL2 arecoupled to source regions 412 and 414, respectively, of transistors 410and 420. A metal trace 460 connects the drain regions of p-channeltransistor 410 and n-channel transistor 420. The source of selecttransistor 450 is electrically coupled to the drains of transistors 410and 420. The gate of select transistor 450 is coupled to receive aselect signal SEL, and the drain of select transistor 450 carries a datasignal RD. The capacitive coupling provided by capacitor structure 100facilitates operation of EEPROM cell 400.

During a read operation, a read input voltage VIN is applied to metal-1trace 180 in capacitor structure 100. The source of p-channel transistor410 (i.e., bit line BL1) is coupled to a positive supply voltage, andthe source of n-channel transistor 420 (i.e., bit line BL2) is coupledto ground. The select signal SEL is activated, thereby turning on selecttransistor 450. During the read operation, transistors 410 and 420 forman inverter circuit having a threshold voltage that depends on thecharge stored by floating gate 150 and the coupling ratio of thecapacitor structure 100. Under these conditions, an output voltage VOUT,representative of the charge stored on patterned polysilicon structure150, is provided on metal trace 460, and is routed through selecttransistor 450 as the read data signal RD.

FIG. 5 is a CMOS inverter transfer curve 500 illustrating the responseof the output voltage VOUT in response to the input voltage VIN, whenmemory cell 400 is programmed, erased or fresh. The CMOS invertertransfer curve 500 is determined by measuring the output voltage VOUTwhile sweeping the input voltage VIN from −1 Volt to 5 Volts. Asillustrated in FIG. 5, if memory cell 400 is fresh (i.e., has not beenprogrammed or erased) or programmed, the inverter has a relatively highthreshold voltage, such that p-channel transistor 410 is initiallyturned on and n-channel transistor is initially turned off. Under theseconditions, the output voltage VOUT is approximately equal to thevoltage on bit line BL1 (i.e., 1 Volt). The output voltage VOUT remainsat 1 Volt until the input voltage VIN is increased to about 3.25 Volts.At this time, n-channel transistor 420 turns on and p-channel transistor410 turns off, thereby causing the output voltage VOUT to become equalto the voltage on bit line BL2 (i.e., 0 Volts).

If memory cell 400 is erased, the inverter has a relatively lowthreshold voltage, such that p-channel transistor is initially turnedoff and n-channel transistor 420 is initially turned on, such that theoutput the output voltage VOUT is approximately equal to the voltage onbit line BL1 (i.e., 1 Volt). The output voltage VOUT remains at 1 Voltuntil the input voltage VIN is increased to about 0 Volts. At this time,n-channel transistor 420 turns on and p-channel transistor 410 turnsoff, thereby causing the output voltage VOUT to become equal to thevoltage on bit line BL2 (i.e., 0 Volts).

The characteristics illustrated in FIG. 5 are typical for a high qualityCMOS inverter, and shows that the control gate coupling ratio is about90%. The same slope of transfer curves for initial, programmed anderased cells further confirms the high quality (i.e., constantcapacitance) of the control gate.

The fabrication of capacitor structure 100 in accordance with oneembodiment of the present invention will now be described. In thisembodiment, capacitor structure 100 is fabricated using a conventional0.18 micron planar CMOS (single-poly) process. This CMOS processincludes both low voltage transistors and high voltage transistors. Forexample, low voltage transistors may have a gate length of 0.18 microns,and high voltage transistors may have a gate length of 0.35 microns.Other conventional processes can be used in other embodiments.

Because the fabrication of capacitor structure 100 is related to thefabrication of the low voltage transistors and the high voltagetransistors typically available in a conventional CMOS process, thefollowing description will also include the fabrication of a low voltagetransistor and a high voltage transistor, which are fabricated on thesame chip as capacitor structure 100.

FIGS. 6A-6D are cross sectional views of capacitor structure 100, a lowvoltage PMOS transistor 610 and a high voltage PMOS transistor 620,during various processing steps, in accordance with one embodiment ofthe present invention. Low voltage PMOS transistor 610, high voltagePMOS transistor 620 and capacitor structure 100, are fabricated inregions 601, 602 and 603, respectively, of substrate 101.

As shown in FIG. 6A, n-type active regions 102, 613 and 623 and fielddielectric regions 105 are formed in p-type substrate 101. N-type activeregions 102, 613 and 623 may be a continuous n-well region or separaten-well regions. The required p-type well regions (not shown) are alsoformed. Capacitor dielectric layer 108, low voltage gate dielectriclayer 614 and high voltage gate dielectric layer 624 are then formedover the upper surface of the resulting structure. In one embodiment,capacitor dielectric layer 108 and high voltage gate dielectric layer624 are both formed from the same high voltage (HV) gate oxide layer.For example, capacitor dielectric layer 108 and high voltage gatedielectric layer 624 may be created by a layer of thermally grownsilicon oxide having a thickness of about 70 Angstroms. Low voltagedielectric layer 614 can be, for example, a layer of thermally grownsilicon oxide having a thickness of about 30 Angstroms. All of theabove-described elements are commonly available in a conventional planarCMOS process.

A single polysilicon layer is deposited over the resulting structure. Aphotoresist mask, which defines the locations of the polysiliconstructures to be formed, is formed over the polysilicon layer. An etchis then performed through this photoresist mask, thereby forming lowvoltage gate electrode 615, high voltage gate electrode 625 andpatterned polysilicon structure 150 (which includes polysilicon traces151-153 as illustrated). As described above in connection with FIGS.1-4, the patterned polysilicon structure 150 may form a floating gate ofa non-volatile memory cell. The photoresist mask is designed such thatlines 151-155 of patterned polysilicon structure 150 each has a width ofabout 0.14 microns, low voltage gate electrode 615 has a width of about0.18 microns, and high voltage gate electrode 625 has a width of about0.35 microns. The photoresist mask is then stripped.

As illustrated in FIG. 6A, another photoresist mask 630 is formed overthe resulting structure. This photoresist mask 630 exposes all regionswhich are to receive a low voltage p-type lightly doped drain (LDD)implant (and covers the remaining regions). In accordance with thedescribed embodiment, regions 601 and 603 are exposed throughphotoresist mask 630, and region 602 is covered by photoresist mask 630.The low voltage p-type LDD implant results in the formation of p-typeLDD regions 611A-612A in region 601, and the formation of p-type LDDregions 631-634 in region 603. The p-type LDD regions 611A and 612A arealigned with low voltage gate electrode 615. Similarly, the p-type LDDregions 631-634 are aligned with patterned polysilicon structure 150.The low voltage p-type LDD implant may be performed, for example, byimplanting implanting BF2 at a dose of about 4E14 cm−2 and an energy ofabout 10 keV. Photoresist mask 630 is then stripped.

As illustrated in FIG. 6B, another photoresist mask 640 is formed overthe resulting structure. This photoresist mask 640 exposes all regionswhich are to receive a high voltage p-type lightly doped drain (LDD)implant (and covers the remaining regions). In accordance with thedescribed embodiment, regions 602 and 603 are exposed throughphotoresist mask 640, and region 601 is covered by photoresist mask 640.The high voltage p-type LDD implant results in the formation of p-typeLDD regions 621A-622A in region 602, and the formation of p-type LDDregions 641-644 in region 603. The p-type LDD regions 621A and 622A arealigned with high voltage gate electrode 625. Similarly, the p-type LDDregions 641-644 are aligned with patterned polysilicon structure 150.The high voltage p-type LDD implant may be performed, for example, byimplanting BF2 at a dose of about 2.5E14 cm−2 and an energy of about 18keV. Photoresist mask 640 is then stripped.

In accordance with one embodiment of the present invention, p-type LDDregions 641-644 include impurities implanted during both the highvoltage p-type LDD implant and the low voltage p-type LDD implant. Thedashed lines in regions 641-644 represent the presence of the impuritiesimplanted during the low voltage p-type LDD implant. The dopantconcentration in regions 641-644 is therefore greater than the dopantconcentration in regions 611A-612A or regions 621A-622A. Although thepresent invention has described the low voltage p-type LDD implant asoccurring before the high voltage p-type LDD implant, this order can bereversed in other embodiments.

As illustrated in FIG. 6C, dielectric sidewall spacers 109, 619 and 629are formed adjacent to patterned polysilicon structure 150, low voltagegate electrode 615 and high voltage gate electrode 625, respectively.Dielectric sidewall spacers 109, 619 and 629 can be formed, for example,by silicon nitride, which is deposited and then etched back in a mannerwell known to those of ordinary skill in the art. Another photoresistmask (not shown) is formed over the resulting structure. Thisphotoresist mask exposes all regions which are to receive a heavilydoped P+ implant (and covers the remaining regions). In accordance withthe described embodiment, regions 601-603 are exposed through this P+photoresist mask. The P+ LDD implant results in the formation of P+ typeregions 611B and 612B in region 601, the formation of P+ type regions621B and 622B in region 602, and the formation of P+ type regions141-144 in region 603. The portions of p-type regions 641-644 that arenot included in P+ type regions 141-144 are re-labeled as p-type regions131-136 in FIG. 6C. Note that P+ type regions 611B and 612B are alignedwith dielectric sidewall spacers 619. Similarly, P+ type regions 621Band 622B are aligned with dielectric sidewall spacers 629. P+ typeregions 141-144 are aligned with dielectric sidewall spacers 109. The P+type implant may be performed, for example, by implanting boron at aconcentration of about 4*1015 atoms/cm−2 and an energy of about 7 keV.The P+ photoresist mask is then stripped.

As illustrated in FIG. 6D, back end processing is subsequentlyperformed, which includes forming metal salicide regions 10-21 (e.g.,CoSi), depositing pre-metal dielectric 170, forming contacts (includingcontacts 161-164) through pre-metal dielectric 170 and capacitordielectric layer 108, and forming a metal-1 layer (which includes metaltrace 180) over the resulting structure. Additional dielectric layersand metal layers (not shown) are typically formed over the metal-1layer.

After the above-described p-type regions have been implanted, a thermaldrive-in cycle anneal is performed, which causes the various p-typeregions 611A, 612A, 611B, 612B, 621A, 622A, 621B, 622B, 131-136 and141-144 to diffuse in the manner illustrated in FIG. 6D. In oneembodiment, the thermal drive-in cycle includes a thermal process at atemperature of about 1020° C. for about 20 seconds. The relatively highdopant concentration of regions 131-136, along with the relativelynarrow width of polysilicon traces 151-153, result in the merging ofthese p-type regions under polysilicon traces 151-153, thereby formingcontinuous p-type region 145. Each of p-type regions 131-136 laterallydiffuses at least about 0.07 microns in the described example.

Note that the p-type implanted regions do not merge in the PMOStransistors 610 and 620, as these transistors are fabricated inaccordance with conventional CMOS processing techniques.

In the foregoing manner, control gate capacitor structure 100 is formedin a conventional CMOS process without adding any process steps.

Advantageously, the continuous p-type region 145 under the patternedpolysilicon structure 150 forms a capacitance (C1) that has a weakdependence on voltage. Because capacitor dielectric layer 108 is formedbefore the continuous p-type region 145, the quality of the capacitordielectric layer 108 (e.g., silicon oxide) is improved with respect tothe prior art, which required that a capacitor dielectric layer beformed over an existing heavily doped diffusion region.

FIGS. 7, 8, 9 and 10 illustrate variations of capacitor structure 100 inaccordance with other embodiments of the present invention. Similarelements in FIGS. 1 and 7-10 are labeled with similar reference numbers.

As illustrated in FIG. 7, capacitor structure 700 eliminates contacts161-164 and metal-1 trace 180 of capacitor structure 100, therebyeliminating capacitive elements C2 and C3, but retaining capacitiveelement C1.

As illustrated in FIG. 8, capacitor structure 800 eliminates continuousp-type region 145 and metal-1 trace 180 of capacitor structure 100,thereby eliminating capacitance C1 and C3, but retaining capacitiveelement C2.

As illustrated in FIG. 9, capacitor structure 900 eliminates narrowpolysilicon trace 154 of capacitor structure 100, and replaces contacts161-164 of capacitor structure 100 with contact structure 160. Contactstructure 160 includes flat plates which are positioned parallel to thewalls of polysilicon traces 151-153, thereby increasing the capacitanceof capacitive element C2.

As illustrated in FIG. 10, capacitor structure 1000 can be created byfabricating the capacitor structure 900 of FIG. 9 over a shallow trenchisolation region 1001. This may effectively reduce the required layoutarea (in terms of the required active region of the substrate) of anassociated non-volatile memory cell.

Although the invention has been described in connection with severalembodiments, it is understood that this invention is not limited to theembodiments disclosed, but is capable of various modifications, whichwould be apparent to a person skilled in the art. For example, althoughthe present invention was described in terms of a continuous p-typeregion 145 fabricated in an n-type active region 102, it is understoodthat these polarities can be reversed in other embodiments. Thus, theinvention is limited only by the following claims.

1. A method comprising: forming one or more transistors in asemiconductor substrate, wherein each of the one or more transistors hasa gate with a width equal to or greater than a first width; forming acapacitor dielectric layer over a first region of the semiconductorsubstrate; forming a patterned polysilicon structure over the capacitordielectric layer, wherein the patterned polysilicon structure includesone or more narrow polysilicon lines having a width less than the firstwidth; and then implanting impurities into the first region of thesemiconductor substrate using the patterned polysilicon structure as amask; performing a thermal-drive in cycle, wherein the implantedimpurities in the first region of the semiconductor diffuse to create acontinuous diffusion region under the patterned polysilicon structure.2. The method of claim 1, wherein the step of forming one or moretransistors comprises: forming a first polysilicon gate having the firstwidth over a second region of the semiconductor substrate; and forming asecond polysilicon gate having a second width, greater than the firstwidth, over a third region of the semiconductor substrate.
 3. The methodof claim 2, wherein the step of implanting impurities into the firstregion of the semiconductor substrate further comprises: performing afirst implant wherein impurities are implanted into the first region andthe second region of the semiconductor substrate, but not the thirdregion of the semiconductor substrate; performing a second implantwherein impurities are implanted into the first region and the thirdregion of the semiconductor substrate, but not the second region of thesemiconductor substrate.
 4. The method of claim 3, wherein theimpurities of the first and second implants have the same conductivitytype.
 5. The method of claim 1, wherein the one or more narrowpolysilicon lines define one or more openings in the patternedpolysilicon structure, the method further comprising: forming apre-metal dielectric material over the patterned polysilicon structure;and forming a first set of one or more contacts extending through thepre-metal dielectric material and the one or more openings in thepatterned polysilicon structure, wherein the first set of one or morecontacts do not physically contact the patterned polysilicon structure.6. The method of claim 5, further comprising forming a conductive traceon the pre-metal dielectric material, wherein the conductive tracecontacts the first set of one or more contacts.
 7. The method of claim5, further comprising forming a non-volatile memory cell that uses thepatterned polysilicon structure as a floating gate.